1. Field of the Invention
The present invention relates to an acoustic transducer and particularly to an acoustic transducer capable of performing acoustic radiation under water.
2. Description of the Related Art
In the case of performing an underwater observation like a marine survey such as an ocean crust survey, light or a radio wave is not used but a sound wave is used. This is because the light or the radio wave is easily attenuated under water but the sound wave is hard to be attenuated even under water. Thus, an acoustic transducer using a vibrator is used as a device for generating a sound wave under water.
There are many kinds of acoustic transducers. One example of the related art is an acoustic transducer using a hollow cylindrical piezoelectric vibrator (for example, “The basis and application of marine acoustics [Kaiyo-onkyo-no-kiso-to-ouyou in Japanese]”, Marine acoustics society of Japan, Seizando-syoten, 2004, pp. 58-60, in Japanese). In this acoustic transducer, a cylindrical piezoelectric vibrator has electrodes provided on the inner and outer surfaces thereof and is polarized in the direction of thickness, that is, between the inner and outer electrodes. Application of a voltage between the inside electrode and the outside electrode generates a breathing vibration, which is made by the cylindrical piezoelectric resonator uniformly transforming inwardly and outwardly in the radial direction. Sound is emitted into liquid from the side surface of the cylindrical piezoelectric vibrator by the use of this breathing vibration. In the case of employing an acoustic transducer of a free-flooded structure, the acoustic transducer emits sound from the side surface thereof and, in addition, emits sound from the inner surface to the liquid in a hollow portion thereof and further emits sound to the outside by the use of the resonance of a water column of the liquid.
Another example of the related art will be described. FIG. 1A is a schematic view of an exterior appearance of a free-flooded type acoustic transducer in which bending vibration modules are arranged in the shape of a cylinder. FIG. 1B is a schematic view of a section along A-A′ in FIG. 1A. Electrodes 104 are arranged on the inner and outer surfaces of plate-shaped piezoelectric vibrator 102 and one surface of electrode 104 is bonded to a vibration plate 103. In this way, bending vibration module 101 is constructed. In the acoustic transducer of this related art, bending vibration modules 10l are arranged in the shape of a cylinder and adjacent bending vibration modules 101 are bonded to each other (for example, Japanese Patent Laid-Open No. 02-238799). Bending vibration module 101 has waterproof structure 110. The respective bending vibration modules 101 are repeatedly bent back and forth in the direction of thickness of bending vibration module 101 to thereby emit sound to the surrounding liquid. Further, bending vibration modules 101 emit sound by the use of the resonance of a water column of the liquid in a cylindrical space surrounded by bending vibration modules 101.
Still another example of the related art will be described. FIG. 2A is a schematic view of the exterior appearance of a barrel stave type acoustic transducer of the related art. FIG. 2B is a schematic view of a section of a bending vibration module of the acoustic transducer shown in FIG. 2A. FIG. 2C is a schematic view of the interior of the acoustic transducer shown in FIG. 2A. Although not shown in the drawing, in reality, the entire outer surface has a waterproof structure.
In the barrel stave type acoustic transducer, as shown in FIG. 2A, a plurality of bending vibration modules 101 are arranged in the shape of a cylinder, and adjacent bending vibration modules 101 are not bonded to each other but have clearance 105 formed between them. Both end portions of bending vibration module 101 are fixed to end plates 106.
Bending vibration module 101, as shown in FIG. 2B, is constructed in such a way that electrodes 104 are bonded to both surfaces of plate-shaped piezoelectric vibrator 102 and such that one surface is bonded to vibration plate 103. Further, as shown in FIG. 2C, end plates 106 are supported by support column 107 so as to prevent the interval between end plates 106 from being changed.
In an acoustic transducer that directly utilizes the vibration of a non-, free-flooded type piezoelectric vibrator (having end portions not opened), that is, a hollow cylindrical piezoelectric vibrator, it is when a resonance vibration is generated in which one wavelength of longitudinal vibration in a circumferential direction of the cylindrical piezoelectric vibrator is coincident with the length of a circumference that the best efficiency of an acoustic emission from a cylindrical piezoelectric vibrator is produced. The speed of sound transmitted through a material constructing a piezoelectric vibrator is generally fast, so that for example, a cylindrical piezoelectric vibrator having a diameter of about 10 cm generates as high a resonance frequency as 5 to 10 kHz. When the frequency is reduced, one wave length is increased. Hence, in order to generate the acoustic emission with high efficiency even in a low frequency, it is preferable to employ a cylindrical piezoelectric vibrator having a larger diameter, which results in enlarging the size of the acoustic transducer.
In the above-mentioned barrel stave type acoustic transducer, which is one example of the related art, a resonance frequency can be reduced by the use of bending vibration. However, this type of acoustic transducer presents the problems in which the clearance between bending vibration modules 101 is restrained in a waterproof structure or in which bending vibration is repressed by hydraulic pressure. Hence, it is difficult to realize this type of acoustic transducer.
Further, in the above-mentioned free-flooded type acoustic transducer, which is another example of the related art, the frequency band of the acoustic emission is widened by the use of two kinds of resonance frequencies due to the breathing vibration and by the resonance of the water column in which a resonance frequency is low. However, the water column resonance frequency is determined by the total length of the acoustic transducer. In the case where the length in the axial direction of the acoustic transducer is about 20 cm, the water column resonance frequency becomes about 1 to 2 kHz. In order to realize a lower resonance frequency than this frequency, the acoustic transducer needs to have a longer length or needs to have a resonant tube (or acoustic tube) provided. Therefore, in order to generate acoustic emission of a low frequency, the acoustic transducer needs to have a larger diameter or a longer length as frequency is reduced.